Wound and themoformed element and method of manufacturing same

Abstract

A heating element for heating a flexible intravenous tube includes a resistance heating wire having a pair of terminal ends encapsulated within an electrically insulating polymeric layer. The polymeric layer and the resistance heating wire are formed into a plurality of turns defining a coil having a central axis. Each turn of the coil is independently elastically expandable to surround a portion of the flexible intravenous tube when the intravenous tube is disposed axially through the coil such that the coil conforms to the shape of the flexible intravenous tube.

Description

FIELD OF THE INVENTION

[0003] This invention relates to electric resistance heating elements, and more particularly, to plastic insulated resistance heating elements containing encapsulated resistance material and shaped to surround tubular structures.

BACKGROUND OF THE INVENTION

[0004] A number of heating tapes have been developed that include resistance heating elements and that are used primarily to heat the exterior surface of tubes, pipes, containers and the like. Good examples of these heating tapes include those described in R. M. Combs, U.S. Pat. No. 2,719,907, issued Oct. 4, 1955; R. W. Logan et al., U.S. Pat. No. 2,710,909, issued Jun. 14, 1955; and G. H. Morey, U.S. Pat. No. 3,268,846, issued Aug. 23, 1966, which are all hereby incorporated by reference herein.

[0005] R. M. Combs '907 discloses a heating tape in the form of a flat elongated strip with a central core sandwiched between two insulating layers. The core consists of a rubber supporting strip that supports a Nichrome (Ni—Cr) wire coiled upon a rayon cord. The wire is arranged in a series of loops extending from lateral edge to lateral edge of the supporting strip and periodically contacting a pair of lead wires. An outer cover of the heating tape is grooved such that the grooves may be interlocked to secure adjacent coils formed when the heating tape is wrapped around a pipe.

[0006] R. W. Logan et al. '909 discloses a heating tape including a plurality of parallel resistance wires connected in series and sandwiched between two sheets of flexible reinforced dielectric material, such as silicone rubber. The dielectric material is covered by two sheets of glass cloth. Logan et al. teaches that the heating tape may be wrapped around a pipe or other tubular body. The tape may then be secured in the wrapped configuration by overlapping an end of the heating tape with a portion of the body of the heating tape.

[0007] G. H. Morey '846 discloses a flexible heating tape including a plurality of ribbon like resistance heating elements arranged in a parallel relationship and sandwiched between two thin plastic strips.

[0008] Plastic welding sleeves have also been developed for use in bonding two fitted thermoplastic pipes together. A good example of a welding sleeve is disclosed in Blumenkranz, U.S. Pat. No. 4,436,999, issued Mar. 13, 1984, the entirety of which is hereby incorporated by reference herein. Blumenkranz '988 discloses a welding sleeve including two electrically conducting wires embedded in a thermoplastic sheath. A length of the welding sleeve is wound into a spiral pattern on a mandrel having substantially the same diameter as an insert pipe of the two pipes to be fused together. Each succeeding turn around the mandrel is wound as closely as possible to the preceding turn, so that there is no space between the turns. The adjacent turns of the welding sleeve are then fused so that they are joined to form a self-sustaining spiral heating element. The sleeve is fitted over the insert pipe, and the insert pipe is then fitted into a second pipe and energized to fuse the two pipes together.

[0009] The heated tape elements described above must all be manually wrapped around an object to be heated, such as a pipe, and then must be affirmatively secured in the wrapped configuration in some manner, such as by overlapping the end of the tape with a portion the heating tape or by interlocking specially designed grooves. The heating tapes are not pre-formed into a configuration that is capable of heating objects, such as pipes, that commonly have different diameters without performing the above-described wrapping and securing steps. The Blumenkranz heating element is pre-formed to accommodate the general shape of a pipe structure and does not need to be wrapped or independently secured to the pipe, but the Blumenkranz heating element is designed for use only with pipes having a diameter substantially matching that of the welding sleeve heating element. Therefore, there remains a need for a heating element that is adaptable to heat a plurality of heat objects of different sizes but that does not need to be wrapped and then secured around the object to be heated.

SUMMARY OF THE INVENTION

[0010] The present invention provides a heating element for heating a flexible intravenous tube and a method of manufacturing the same. The heating element comprises a resistance heating wire having a pair of terminal ends. The resistance heating wire is encapsulated within an electrically insulating polymeric layer. The polymeric layer and the resistance heating material are formed into a plurality of turns defining a coil having a central axis. Each of the turns is independently elastically expandable to surround a portion of the intravenous tube when the intravenous tube is disposed axially through the coil such that the coil conforms to the shape of the flexible intravenous tube.

[0011] The inventors of the heating element believe that the heating element may be of great benefit in several medical intravenous applications. For example, several dies used in angioplasty procedures are very viscous at room temperature. The timing of the injection of these dies must often be quite precise, such as timing the injection of the die to occur in between heart beats. These dies are often stored at reduced temperatures, and it is believed that heating the die along the length of the tube may reduce the viscosity of the die, thereby aiding in the control of the timing of the injection. The heating element may also be used to raise and regulate the temperature of other intravenous fluids, such as blood plasma, from room temperature, or below room temperature, to body temperature in a controlled manner (i.e., without overheating the blood plasma and destroying the red blood cells) during procedures such as transfusions. These intravenous fluids are often refrigerated prior to infusion to prevent incubation of harmful organisms. Infusion of the fluid at these reduced temperatures can induce thermal shock in a patient and resulting in possible patient mortality. These heating applications may be accomplished by the heating element of the present invention, all while avoiding the shortcomings of prior art heating elements that may be used to heat other tubular structures. There is no need to manually wrap the heating element, coil by individual coil, around the intravenous tube. The intravenous tube may simply be inserted along the central axis of the coiled heating element. There is also no need to independently secure the heating element around the tube because the heating element is self-securing and conforms to the shape of, and frictionally engages, the flexible intravenous tube.

[0012] The utility of the present invention also extends past the heating of intravenous tubes to the heating of other liquids or devices, such as oil or fuel lines, food and beverage service applications, etc. The present invention provides an expandable heating element comprising a resistance heating material surrounded by an electrically insulating polymeric layer. The polymeric layer and the resistance heating material are formed into a plurality of turns defining a coil having an original diameter and a central axis. A plurality of the turns of the coil are independently elastically expandable to a diameter greater than the original diameter of the coil to surround a cylindrical body disposed axially through the coil where at least a portion of the cylindrical body has a diameter greater than the original diameter of the coil.

[0013] The above and other features of the present invention will be better understood from the following detailed description of the preferred embodiments of the invention that is provided in connection with the accompanying drawings.

A BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The accompanying drawings illustrate preferred embodiments of the invention, as well as other information pertinent to the disclosure, in which:

[0015] FIG. 1 is a front plan view of a resistance heating element, including a resistance wire disposed in a circuit path on a supporting substrate and joined to a pair of electrical connectors;

[0016] FIG. 1A is a front plan, enlarged view, of a portion of the resistance heating element of FIG. 1, showing the preferred cross-stitch attachment to the supporting substrate;

[0017] FIG. 2 is a rear plan view of the resistance heating element of FIG. 1;

[0018] FIG. 3 is a front perspective view of a preferred programmable sewing machine and computer for manufacturing resistance heating elements;

[0019] FIG. 4 is a top plan view of a heated element assembly including a resistance heating element according to the present invention;

[0020] FIG. 5 is a cross-sectional view of the heated element assembly of FIG. 4 taken along lines 1-1;

[0021] FIG. 6 is a cross-sectional view of a multilayered heated element assembly according to the present invention;

[0022] FIG. 7 a is a top plan view of a tubular shaped thermoplastic body for providing thermoplastic sheets according to the present invention;

[0023] FIG. 7 b is a side elevational view of a tubular shaped thermoplastic body for providing thermoplastic sheets according to the present invention;

[0024] FIG. 8 is a partial, side elevational view of an exemplary heating element according to the present invention;

[0025] FIG. 9 is a front elevational view of the heating element of FIG. 8;

[0026] FIG. 10 is an enlarged, cross-sectional view of the heating element of FIG. 8 taken along lines 10-10;

[0027] FIG. 11 is a partial, side elevational view of another embodiment of an exemplary heating element according to the present invention;

[0028] FIG. 12 is a rear elevational view of the heating element of FIG. 11;

[0029] FIG. 13 is a top plan view of a planar resistance heating element that may be formed into the heating element of FIG. 11;

[0030] FIG. 13A is a top plan view of the planar resistance heating element of FIG. 13 shown with top polymeric layer removed;

[0031] FIG. 14 is a front elevational view of the planar resistance heating element of FIG. 13;

[0032] FIG. 15A is an enlarged, cross-sectional view of an exemplary resistance heating element including a plurality of resistance heating wires that may be formed into the heating element of FIG. 8;

[0033] FIG. 15B is a cross-sectional view of another exemplary planar resistance heating element that may be formed into the heating element of FIG. 11;

[0034] FIG. 16 is a partial, side elevational view of a heating element according to the present invention having two sets of interconnected parallel turns;

[0035] FIG. 17 is a partial, perspective view of the heating element of FIG. 11 with individual turns expanded to surround a pipe having diameter greater than the original diameter of the heating element;

[0036] FIG. 18 is a partial, perspective view of a flexible intravenous tube disposed axially through the heating element of FIG. 8; and

[0037] FIG. 19 is a cross-sectional view of an exemplary thermoplastic elastomer resistance heating element including reinforcing substrates.

DETAILED DESCRIPTION OF THE INVENTION

[0038] As used herein, the following terms are defined:

[0039] “Laminate” means to unite laminae via bonding them together, usually with heat, pressure and/or adhesive. It normally is used to refer to flat sheets, but also can include rods and tubes. The term refers to a product made by such bonding;

[0040] “Serpentine Path” means a path which has one or more curves for increasing the amount of electrical resistance material in a given volume of polymeric matrix, which is useful in controlling the thermal capacity and expansion of the element;

[0041] “Melting Temperature” means the point at which a fusible substance begins to melt;

[0042] “Melting Temperature Range” means the temperature range over which a fusible substance starts to melt and then becomes a liquid or semi-liquid;

[0043] “Degradation Temperature” means the temperature at which a thermoplastic begins to permanently lose its mechanical or physical properties because of thermal damage to the polymer's molecular chains;

[0044] “Evacuating” means reducing air or trapped air bubbles by, for example, vacuum or pressurized inert gas, such as argon, or by bubbling the gas through a liquid polymer.

[0045] “Fusion Bond” means the bond between two fusible members integrally joined, whereby the polymer molecules of one member mix with the molecules of the other. A Fusion Bond can occur, even in the absence of any direct or chemical bond between individual polymer chains contained within said members;

[0046] “Fused” means the physical flowing of a material, such as ceramic, glass, metal or polymer, hot or cold, caused by heat, pressure or both;

[0047] “Electrofused” means to cause a portion of a fusible material to flow and fuse by resistance heating;

[0048] “Stress Relief” means reducing internal stresses in a fusible material by raising the temperature of the material or material portion above its stress relief temperature, but preferably below its Heat Deflection Temperature; and

[0049] “Flap” or “Flap Portion” means a portion of an element which can be folded without damaging the element structure.

Resistance Heating Element

[0050] With reference to the Figures, and particularly FIGS. 1, 1A and 2 thereof, there is shown a first embodiment of a resistance heating element 10 having a diameter of about 11 cm. The preferred resistance heating element 10 may include a regulating device for controlling electric current. Such a device can include, for example, a thermistor, or a thermocouple, for preventing overheating of the polymeric materials disclosed in this invention. The resistance heating elements 10 of this invention can take on any number of shapes and sizes, including squares, ovals, irregular circumference shapes, tubes, cup shapes and container shapes. Sizes can range from less than one inch square to 21 in.×26 in. with a single sewing operation, and greater sizes can be available if multiple elements are joined together. Greater sizes are also available with continuous sewing where a substrate is fed from a roll of substrate.

[0051] As shown in FIG. 1, the resistance heating element 10 includes a resistance wire 12 disposed in a helical pattern or circuit path 18. The ends of the resistance wire 12 are generally riveted, grommeted, brazed, clinched, compression fitted or welded to a pair of electrical connectors 15 and 16. One circuit path is illustrated in FIGS. 1 and 2. The circuit includes a resistance heating material, which is ideally a resistance heating wire 12 wound into a serpentine path containing about 3-200 windings, or, a resistance heating material, such as ribbon, a foil or printed circuit, or powdered conducting or semi-conducting metals, polymers, graphite, or carbon, or a conductive coating or ink. More preferably the resistance heating wire 12 includes a Ni—Cr alloy, although certain copper, steel, and stainless-steel alloys could be suitable. A positive temperature coefficient wire (“PTC”), which is self-regulating, may also be suitable. The resistance heating wire 12 can be provided in separate parallel paths, or in separate layers. Whatever material is selected, it should be electrically conductive, and heat resistant.

Substrates

[0052] As used herein, the term “supporting substrate” refers to the base material on which the resistance material, such as wires, are applied. The supporting substrate 11 of this invention should be capable of being pierced, penetrated, or surrounded, by a sewing needle for permitting the sewing operation. Other than this mechanical limitation, the substrates of this invention can take on many shapes and sizes. Flat flexible substrates are preferably used for attaching an electrical resistance wire with a thread. Non-plastic materials, such as glasses, semiconductive materials, and metals, can be employed so long as they have a piercable cross-sectional thickness, e.g., less than 10-20 mil, or a high degree of porosity or openings therethrough, such as a grid, scrim, woven or nonwoven fabric, for permitting the sewing needle of this invention to form an adequate stitch. The supporting substrate 11 of this invention need not necessarily contribute to the mechanical properties of the final heating element, but may contain high strength fibers. Such fibers could contain carbon, glass, aramid fibers melt-bonded or joined with an adhesive to form a woven or non-woven mat. Alternatively, the supporting substrate 11 of this invention may contain ordinary, natural, or synthetic fibers, such as cotton, wool, silk, rayon, nylon, polyester, polypropylene, polyethylene, etc. The supporting substrate may also comprise a synthetic fiber such as Kevlar or carbon fibers that have good thermal uniformity and strength. The advantage of using ordinary textile fibers, is that they are available in many thicknesses and textures and can provide an infinite variety of chemistry, porosity and melt-bonding ability. The fibers of this invention, whether they be plastic, natural, ceramic or metal, can be woven, or spun-bonded to produce non-woven textile fabrics.

[0053] Specific examples of supporting substrates 11 useful in this invention include non-woven fiberglass mats bonded with an adhesive or sizing material such as model 8440 glass mat available from Johns Manville, Inc. Additional substrates can include polymer impregnated fabric organic fabric weaves, such as those containing nylon, rayon, or hemp etc., porous mica-filled plate or sheet, and thermoplastic sheet film material. In one embodiment, the supporting substrate 11 contains a polymeric resin which is also used in either the first thermoplastic sheet 110 or second thermoplastic sheet 105, or both of a heated element assembly 100 described below. Such a resin can be provided in woven or non-woven fibrous form, or in thin sheet material having a thickness of 20 mil. or less. Thermoplastic materials can be used for the supporting substrate 11 which will melt-bond or liquefy with the thermoplastic sheets 110, 105, so as to blend into a substantially uniform structure.

Sewing Operation

[0054] With reference to FIG. 3, the preferred programmable sewing machine 20 will now be described. The preferred programmable sewing machine is one of a number of powerful embroidery design systems that use advanced technology to guide an element designer through design creation, set-up and manufacturing. The preferred programmable sewing machine 20 is linked with a computer 22, such as a personal computer or server, adapted to activate the sewing operations. The computer 22 preferably contains or has access to, embroidery or CAD software for creating thread paths, borders, stitch effects, etc.

[0055] The programmable sewing machine 20 includes a series of bobbins for loading thread and resistance heating wire or fine resistance heating ribbon. Desirably, the bobbins are prewound to control tension since tension, without excessive slack, in both the top and bottom bobbins is very important to the successful capturing of resistance heating wire on a substrate. The thread used should be of a size recommended for the preferred programmable sewing machine. It must have consistent thickness since thread breakage is a common mode of failure in using programmable sewing machines. An industrial quality nylon, polyester or rayon thread is highly desirable. Also, a high heat resistant thread may be used, such as a Kevlar thread or Nomex thread known to be stable up to 500° F. and available from Saunders Thread Co. of Gastonia, N.C.

[0056] The programmable sewing machine of this invention preferably has up to 6-20 heads and can measure 6 foot in width by 19 feet long. The sewing range of each head is about 10.6 inches by 26 inches, and with every other head shut off, the sewing range is about 21 inches by 26 inches. A desirable programmable sewing machine is the Tajima Model No. TMLG116-627W (LT Version) from Tajima, Inc., Japan.

[0057] The preferred method of capturing a resistance heating wire 12 onto a supporting substrate 11 in this invention will now be described. First, an operator selects a proper resistive element material, for example, Ni—Cr wire, in its proper form. Next, a proper supporting substrate 11, such as 8440 glass mat, is provided in a form suitable for sewing. The design for the element is preprogrammed into the computer 22 prior to initiating operation of the programmable sewing machine 20. As with any ordinary sewing machine, the programmable sewing machine 20 of this invention contains at least two threads, one thread is directed through the top surface of the supporting substrate, and the other is directed from below. The two threads are intertwined or knotted, ideally somewhere in the thickness of the supporting substrate 11, so that one cannot view the knot when looking at the stitch and the resulting resistance heating element 10. As the top needle penetrates the substrate 11 and picks up a loop of thread mechanically with the aid of the mechanical device underneath, it then pulls it upward toward the center of the substrate 11 and if the substrate is consistent and the thread tension is consistent, the knots will be relatively hidden. In a preferred embodiment of this invention, the resistance heating wire 12 is provided from a bobbin in tension. The preferred programmable sewing machine 20 of this invention provides a third thread bobbin for the electrical resistance wire 12 so that the programmable sewing machine 20 can lay the resistance wire 12 down just in front of the top needle. The preferred operation of this invention provides a zig zag or cross stitch, as shown in FIG. 1A, whereby the top needle crisscrosses back and forth as the supporting substrate 11 is moved, similar to the way an ornamental rope is joined to a fabric in an embroidery operation. A simple looping stitch with a thread 14 is also shown. Sewing by guiding the top needle over either side of the resistance heating wire 12 captures it in a very effective manner and the process is all computer controlled so that the pattern can be electronically downloaded into the computer 22 and automatically sewn onto the substrate of choice.

[0058] The programmable sewing machine 20 can sew an electrical resistance wire 12, 5 mil-0.25 inch in diameter or thickness, onto a supporting substrate 11 at a rate of about 10-500 stitches per minute, saving valuable time and associated cost in making resistance heating elements.

[0059] The ability to mechanically attach resistive elements, such as wires, films and ribbons, to substrates opens up a multitude of design possibilities in both shape and material selection. Designers may mix and match substrate materials by selecting their porosity, thickness, density and contoured shape with selected resistance heating materials ranging in cross-section from very small diameters of about 5 mil to rectangular and irregular shapes, to thin films. Also, secondary devices such as circuits, including microprocessors, fiberoptic fibers or optoelectronic devices, (LEDs, lasers) microwave devices (power amplifiers, radar) and antenna, high temperature sensors, power supply devices (power transmission, motor controls) and memory chips, could be added for controlling temperature, visual inspection of environments, communications, and recording temperature cycles, for example. The overall thickness of the resistance heating element is merely limited by the vertical maximum position of the needle end, less the wire feed, which is presently about 0.5 inches, but may be designed in the future to be as great as 1 inch or more. Resistive element width is not nearly so limited, since the transverse motion of the needle can range up to a foot or more.

[0060] The use of known embroidery machinery in the fabrication of resistance heating elements allows for a wide variety of raw materials and substrates to be combined with various resistance heating materials. The above construction techniques and sewing operation also provide the ability to manufacture multi-layered substrates, including embedded metallic and thermally conductive layers with resistance wires wrapped in an electrically insulating coating, so as to avoid shorting of electric current. This permits the application of a resistance heating wire to both sides of the thermally conductive metallic layer, such as aluminum foil, for more homogeneously distributing resistance heat.

Thermoplastic Laminate Heated Element Construction

[0061] FIG. 4 is a top plan view of a heated element assembly 100 according to the present invention. The heated element assembly 100 includes a first thermoplastic sheet 110 and a second thermoplastic sheet 105 laminated to the first thermoplastic sheet 110. For illustrative purposes, second thermoplastic sheets 105 is shown partially removed from first thermoplastic sheet 110. A resistance heating element 10, described above, is laminated between and to the first and second thermoplastic sheets 110, 105 such that the thermoplastic sheets 110, 105 substantially encompass the circuit path 18, which includes resistance wire 12.

[0062] The supporting substrate of the resistance heating element 10 is preferably not thicker than 0.05 inch, and more preferably 0.025 inch. The supporting substrate should be flexible, either under ambient conditions or under heat or mechanical stress, or both. A thin semi-rigid heated element assembly 100 allows for closer proximity of the resistance heating wire 12 to an object to be heated when the heated element assembly is formed into a final element assembly, such as a combination containment bag and heater. Thin element assemblies according to the present invention provide the flexibility to choose materials with lower RTI (Relative Thermal Index) ratings because less heat needs to be generated by the resistance heating element 10 to provide heat to the outer surfaces of the heating element assembly 100.

[0063] The thermoplastic sheets 110, 105 are laminiated to each other to secure resistance heating element 10 and to form a reformable continuous element structure. The thermoplastic sheets 110, 105 may be heated and compressed under sufficient pressure to effectively fuse the thermoplastic sheets together. A portion of this heat may come from energizing the resistance heating element 10. Alternatively, a resistance heating element 10 may be placed within a bag-shaped thermoplastic body (not shown) where the top layer of the bag may be considered a thermoplastic sheet and the bottom layer of the bag may be considered a thermoplastic sheet (e.g., two thermoplastic sheets secured along mating edges, but providing an opening for insertion of the resistance heating element 10). Air from within the bag may be evacuated, e.g., by pulling a vacuum, thereby collapsing the bag around the resistance heating element 10, and then heat and/or pressure may be applied to the collapsed structure to create a single heated element assembly 100. Also, heated element assembly 100 may be formed by extruding a tubular shaped thermoplastic body 107 (FIGS. 7a and 7b), disposing a resistance heating element 10 within the thermoplastic body 107, and heating and compressing the body 107, particularly along edges 108, to secure the heating element 10 within the thermoplastic body. Regardless of the initial form the thermoplastic sheets take, thermoplastic sheets are preferably laminated such that a flexible continuous element structure is created, including a resistance heating element 10 and preferably with little air trapped between the thermoplastic sheets.

[0064] Preferred thermoplastic materials include, for example: fluorocarbons, polypropylene, polycarbonate, polyetherimide, polyvinylidine fluoride, polyether sulphone, polyaryl-sulphones, polyimides, and polyetheretherkeytones, polyphenylene sulfides, polyether sulphones, and mixtures and co-polymers of these thermoplastics. It is further understood that, although thermoplastic plastics are most desirable for fusible layers because they are generally heat-flowable, some thermoplastics, notably polytetraflouroethylene (PTFE) and ultra high-molecular-weight polyethylene (UHMWPE) do not flow under heat alone. Also, many thermoplastics are capable of flowing without heat, under mechanical pressure only.

[0065] Good results were found when forming a heated element assembly under the conditions indicated in TABLE 1 as follows:

TABLE
	THICKNESS
	OF SHEET	PRESSURE	TIME	TEMP.
MATERIAL	(mil)	(PSI)	(minutes)	(° F.)
Polypropylene	9	22	10	350
Polycarbonate	9	22	10	380
Polysulfone	19	22	15	420
Polyetherimide	9	44	10	430
Polyethersulfone	9	44	10	460
where no vacuum was pulled, “thickness” is the thickness of the thermoplastic sheets in mils (1 mil = .025 mm = .001 inch), “pressure” represents the amount of pressure applied to the assembly during lamination, “temperature” is the temperature applied during lamination, and “time” is the length of time that the pressure and heat were applied.

[0066] The first and second thermoplastic sheets 110, 105 and resistance heating element 10 of the heated element assembly 100 may also be laminated to each other using an adhesive. In one embodiment of the present invention, an ultraviolet curable adhesive may be disposed between the resistance heating element 10 and the first thermoplastic sheet 110 and between the resistance heating element 10 and the second thermoplastic sheet 105, as well as between areas of the thermoplastic sheets 110, 105 which are aligned to be in direct contact. An ultraviolet curable adhesive may be used that is activated by ultraviolet light and then begins to gradually cure. In this embodiment of the present invention, the adhesive may be activated by exposing it to ultraviolet light before providing the second of the thermoplastic sheets 110, 105. The thermoplastic sheets 110, 105 may then be compressed to substantially remove any air from between the sheets 110, 105 and to secure resistance heating element 10 between the thermoplastic sheets 110, 105.

[0067] FIG. 6 illustrates that a heated element assembly 100a according to the present invention may include a plurality of heated layers. A second resistance heating element 10a may be laminated between and to thermoplastic sheet 110 and a third thermoplastic sheet 115.

[0068] The thicknesses of thermoplastic sheets 110, 105 and the thickness of supporting substrate 11 and resistance heating material 12 are preferably selected to form a reformable continuous element structure that maintains its integrity when the element is formed into a final element structure. The heated element assembly 100 according to the present invention, then, is a semi-rigid structure in that it may be reformed, such as by simply folding or folding under heat, pressure, or a combination thereof as required by the chosen thermoplastics, into a desired shape without sacrificing the integrity of the structure.

[0069] Similarly, one or both of the thermoplastic sheets of a heated element assembly 100 or heated element assembly 500 may be coated with a thermally conductive coating that promotes a uniform heat plane on the heated element assembly. An example of such a coating may be found on anti-static bags or Electrostatic Interference (ESI) resistive bags used to package and protect semiconductor chips. Also, thermally conductive, but preferably not electrically conductive, additive may be added to the thermoplastic sheets to promote heat distribution. Examples of such additive may be ceramic powders, such as, for example, Al2O3, MgO, ZrO2, boron nitride, silicon nitride, Y2O3, SiC, SiO2, TiO2, etc. A thermally conductive layer and/or additive is useful because a resistance wire typically does not cover all of the surface area of a resistance heating element 10.

Wound and Thermoformed Element Construction

[0070] For purposes of this description, figures are not drawn to scale. FIGS. 8-10 illustrate an exemplary heating element 400 for heating a flexible intravenous tube. The heating element 400 includes a resistance heating wire 402, or plurality of resistance heating wires 402 (as shown in FIG. 16A), encapsulated within an electrically insulating polymeric layer 404. The polymeric layer 404 and resistance heating wire 402 are formed into a plurality of turns 408 defining a coil having a central axis 410.

[0071] The terminal ends 406 of the resistance heating wire 402 may be coupled to a pair of electrical connectors (not shown) using known techniques such as riveting, grommeting, brazing, clinching, compression fitting, or welding. The electrical connectors may then be coupled to a power source.

[0072] The electrically insulating polymeric layer 404 may comprise a thermoplastic, thermoplastic elastomer or thermosetting material as long as each turn 408 is independently elastically expandable along axes of elastic expansion 412, 414 to surround a portion of an intravenous tube (not shown) when the intravenous tube is disposed axially through the coil along central axis 410. The coil conforms to the shape of the flexible intravenous tube, as shown in FIG. 18, and may be bent, coiled or otherwise manipulated along with flexible intravenous tube 700 during the normal use of the flexible intravenous tube 700.

[0073] Preferred thermoplastic materials are described above in connection with assembly 100. Some preferred thermosetting polymers include epoxies, phenolics, and silicones. Preferred thermoplastic elastomers include the Advanced Polymer Alloys ALCRYN series of products, particularly ALCRYN No. 4080.

[0074] The resistance heating wire 402 may be surrounded by the polymeric layer 404 in several ways. Polymeric layer 404 may be provided as a tubular sheath, and the resistance heating wire 402 may be fed axially through the tubular sheath. Alternatively, the polymeric layer 404 may be extruded over the resistance heating wire(s) 402. If the polymeric layer 404 is extruded over the resistance heating wire 402, polymeric layer 404 may need to be partially stripped from the ends of heating element 400 to expose the terminal ends 406 of the resistance heating wire 402. In one embodiment of an exemplary heating element 400, a ground conductor such as a copper wire or mesh may be wrapped around the polymeric layer 404, and a second polymeric layer can surround the ground conductor to provide a grounded heating element.

[0075] An exemplary electrically insulating polymeric layer 404 includes a PTFE film having a thickness of approximately 0.008-0.010″ extruded around resistance heating wire 402 or a polypropylene tube of the same thickness surrounding the resistance heating wire 402. An exemplary resistance heating wire 402 comprises a Nichrome (80Ni-20Cr) wire or other alloy used in resistive heating.

[0076] Once the resistance heating wire 402 is encapsulated within the polymeric layer 404, the polymeric layer 404 and resistance heating wire 402 may be formed into a plurality of turns 408 by wrapping the polymeric layer 404 and resistance heating wire 402 around a mandrel (not shown) to form the coil shape shown in FIG. 8. The wrapping step may be accomplished manually or as a part of a continuous, automated process. The polymeric layer 404 and the resistance heating wire 402 are heated and then cooled to maintain the coil shape formed by wrapping the polymeric layer 404 and resistance heating wire 402 around the mandrel. In this thermoforming process, the polymeric layer 404 and resistance heating wire may be heated prior to being wrapped around the mandrel, or the mandrel may provide the heat source. Cooling the polymeric layer and resistance heating wire may occur passively, i.e., by air cooling or through contact with the mandrel if the mandrel does not supply the heat source, or the cooling step may occur within an active cooling stage, e.g., within a cold water circulation stage.

[0077] Regardless of the heating and cooling sources, the polymeric layer 404 and resistance heating wire 402 are formed into a plurality of turns 408 defining a coil having a central axis 410, as shown in FIG. 8. The diameter of the mandrel is preferably the same size as or smaller than the outer diameter of the preferred flexible intravenous tube that is to be heated by the heating element 400. It should be apparent that the interior diameter, designated D, of the coil is substantially the same as the diameter of the mandrel. In this manner, the heating element 400 is capable of securing itself around a flexible intravenous tube having the same or larger diameter as the heating element 400 when the flexible intravenous tube is disposed axially through the heating element 400. Because each turn 408 is independently elastically expandable, each turn 408 locally secures itself to a portion of the intravenous tube 700, thereby allowing the coil as a whole to substantially conform to the shape of the intravenous tube 700. Further, because the heating element 400 is thermoformed into the coil shape, the heating element 400 does not rely upon the resistance heating wire 402 to provide structural stability to the heating element 400 or to maintain the coiled shape.

[0078] In one exemplary embodiment, the heating element 400 may include a plurality of sets of interconnected parallel turns. For example, as shown in the partial, side elevational view of the heating element shown in FIG. 16, a first length of the polymeric layer 404 and resistance heating wire 402 preferably comprises approximately half of the total length of the polymeric layer 404 and resistance heating wire 402. This first length may be wrapped around a mandrel to form a first set of turns 408a that terminates at a first terminal end 406a. The remaining length of polymeric layer 404 and resistance heating wire 402 shares a common origination point with the first length. This length of polymeric layer 404 and resistance heating wire 402 may then be wrapped around the mandrel to form a second set of turns 408b that is interconnected with and parallels the first set of turns 408a while terminating at a second one of the terminal ends 406b. An even number of sets of turns provides the advantage of terminal ends 406 that are close in proximity, thereby simplifying coupling the terminal ends 406 to a power source (not shown).

[0079] Referring to the enlarged, cross-sectional view of FIG. 15A, a heating element 400 may include a plurality of resistance heating wires 402. A heating element 400 of a desired length, e.g., the length of the flexible intravenous tube to be heated, may be separated from an extended length of heating element 400, and the ends 406 of the heating element may be stripped to expose the terminal ends of the resistance heating wires 402. These terminal ends may then be connected in series or in parallel depending upon the desired heat characteristics for the heating element 400. The wires 402 may be connected by twisting the wires, soldering the wires, or by other coupling techniques known to those familiar with manufacturing resistance heating elements.

[0080] FIG. 11 is a partial, side elevational view of another exemplary expandable heating element 500. The heating element 500 includes a resistance heating material surrounded by an electrically insulating polymeric layer. The resistance heating material may be a resistance heating wire 504, as shown in FIGS. 11, 13, 13A and 14, but other forms of resistance heating materials may be utilized, such as those described above in connection with assembly 100. The resistance heating material may also be attached to a supporting substrate, such as the 8440 glass mat via a sewing operation as described above. The circuit path formed by the resistance heating material may be grounded or ungrounded as the application requires.

[0081] Still further, the resistance heating material may comprise a woven or non-woven fibrous structure, with or without a supporting substrate, including conductive fibers or fibers coated with a conductive material such as graphite or carbon. The resistance heating layer may also comprise a conductive polymeric resistance heating layer comprising a polymeric layer having conductive additives, such as graphite or carbon fibers or fibers coated with a conductive material, that permit electricity to conduct across or through the film and generates heat when energized. Exemplary fibrous and polymeric resistance heating layers are described in U.S. patent application Ser. No. [D1349-00068] to Ted Von Arx, et al., entitled “Packaging having self-contained heater,” filed on Feb. 12, 2001, the entirety of which is hereby incorporated herein by reference.

[0082] The resistance heating material is surrounded by an electrically insulating polymeric layer preferably by laminating the resistance heating material between two sheets 512a, 512bof an electrically insulating polymeric material to form a planar element 550. Alternatively, the polymeric layer may comprise a film extruded over the resistance heating material. The polymeric layer may comprise a thermoplastic or thermoset material as described above in connection with polymeric layer 404. In any case, an exemplary expandable heating element 500 has a thickness “T,” shown in FIGS. 10 and 12, ranging from 0.010-0.250′. The appropriate thickness is dictated in part by the RTI rating of the polymeric material and the voltage source. The sheets 512a, 512b may be configured such that they have different thermal characteristics. For example, a sheet 512a or 512b which is oriented to be in contact with the tube to be heated may be thinner than the other sheet and/or include thermally conductive additives in order to effectively bias generated heat toward the tube and away from the environment surrounding the tube. If a polymeric layer is extruded over the resistance heating material, the resistance heating material may be disposed off-center such that it is closer to a surface that contacts a tube that is to be heated.

[0083] The resistance heating material may comprise a plurality of resistance heating wires 504 as shown in FIG. 15B. The resistance heating wires 504 and polymeric layer may be formed into continuous sheets that may be used to form the expandable heating element 500. The terminal ends 502 of the wires 504 may be exposed and coupled as desired, e.g., in series or in parallel. A pair of cold pins may also be affixed to the terminal ends, the cold pins may then be coupled to an appropriate power supply.

[0084] FIGS. 11, 13, 13A and 14 show a single resistance heating wire 504 secured between two polymeric sheets 512a, 512b. The resistance heating wire 504 is shown oriented in a simple “u” shaped circuit path, but it should be apparent that the wire 504 may taken on any number of circuit paths, such as a zig-zag or serpentine pattern, in order to occupy more surface area of the final expandable heating element 500.

[0085] Once the resistance heating material is surrounded by the electrically insulating polymeric layer, the polymeric layer and resistance heating material may be formed into a plurality of turns 510 defining a coil having an original diameter, designated as “X,” and a central axis 514. The electrically insulating polymeric layer and resistance heating material of the expandible heating element 500 may be formed into the plurality of turns 510 by wrapping the polymeric layer and resistance heating material around a mandrel and thermoforming the polymeric layer and resistance heating material into the coil shape of FIG. 9 by heating and then cooling the polymeric layer and resistance heating material as described above.

[0086] Turns 510 are each independently elastically expandable along axes of elastic expansion 506, 508 to diameters greater than the original diameter X of the expandable heating element 500 when a cylindrical body having diameter greater than the original diameter X is disposed axially along central axis 514. The heating element 500, therefore, is particularly adapted to heat cylindrical or tubular bodies, such as pipes and dispensing tubes used in the food and medical industries or fuel lines in the automotive industry, to name a few. Also, because each turn 510 is independently elastically expandable, the heating element 500 may be used to heat irregularly shaped articles, such as pipes having diameters that vary along a central axis, pipes that are bent at angles, and the like. Further, an individual heating element 500 formed on a single mandrel may be used to heat a plurality of pipes each having different outer diameters.

[0087] FIG. 17 is a perspective view of a pipe 600 disposed axially along the central axis 514 of a heating element 500. The pipe 600 has an outer diameter “Y” that is greater than the original diameter X of the expandable heating element 500. Each turn 510 is shown independently expanded to surround a different portion of the pipe 600. A heating element 500 is preferably used to heat pipes having diameters ranging from the original diameter X of the heating element 500 to approximately three times the original diameter X. The heating element 500, however, may be used to heat objects having diameters even greater than three times the original diameter X, but it should be understood that when a cylindrical object is disposed axially through the heating element along the central axis 514, the heating element 500 at least partially uncoils to accommodate the larger diameter of the object, and the distance, designated A, separating adjacent turns 510 on each side of the object increases. This increase in turn separation distance A increases the amount of surface area of the object that is not in direct contact with the heating element 500 and, therefore, that is not directly heated by the turns 510 of the heating element 500. It should also be apparent that the length of the heating element 500 decreases as each turn expands to surround an object. This increase in separation distance A and decrease in element length can be accommodated through selection of an appropriate original diameter X and heating element length.

[0088] FIG. 19 is a cross-sectional view of another planar element structure 550b which may be formed into an expandable heating element. If the electrically insulating polymeric layer include thermoplastic elastomer materials, such as thermoplastic elastomer layers 810, 812, one or both of the thermoplastic elastomer layers 810, 812 may further include a reinforcing substrate layer 804, 806. An exemplary reinforcing substrate layer includes an 8440 glass mat as described above fused with the thermoplastic elastomer layers 810, 812, such as during lamination. Because thermoplastic elastomer materials tend to stress relieve more easily than other exemplary thermoplastic materials, the reinforcing substrate which bonds with the thermoplastic elastomer layer provides additional mechanical rigidity. This added rigidity enables the expandable heating element to better secure itself to a tube or pipe when it is expanded or uncoiled to surround the tube or pipe. As described above, the resistance heating material may include a resistance heating wire 802 sewn to a supporting substrate 808.

[0089] Although various embodiments have been illustrated, this was for the purpose of describing, but not limiting the invention. Various modifications which will become apparent to one skilled in the art, are within the scope of this invention described in the attached claims.

Claims

1. A heating element for heating a flexible intravenous tube, said heating element comprising: a resistance heating wire having a pair of terminal ends, said resistance heating wire encapsulated within an electrically insulating polymeric layer, said polymeric layer and said resistance heating wire being formed into a plurality of turns defining a coil having a central axis, wherein each of said turns is independently elastically expandable to surround a portion of said intravenous tube when said intravenous tube is disposed axially through said coil such that said coil conforms to the shape of said flexible intravenous tube.

2. The heating element of claim 1, wherein said coil includes at least two sets of interconnected parallel turns.

3. The heating element of claim 2, wherein said coil includes two sets of interconnected parallel turns, each of said sets terminating at a different one of said terminal ends.

4. The heating element of claim 1, wherein said polymeric layer is a tubular thermoplastic sheath and said resistance heating wire is disposed axially through said tubular thermoplastic sheath.

5. The heating element of claim 1, wherein said polymeric layer is extruded over said resistance heating wire.

6. The heating element of claim 5, wherein said heating element includes a plurality of resistance heating wires connected in series.

7. The heating element of claim 5, wherein said heating element includes a plurality of resistance heating wires connected in parallel.

8. The heating element of claim 1, further comprising a flexible intravenous tube disposed axially through said coil.

9. A method of manufacturing a heating element for a flexible intravenous tube, comprising the steps of: providing a resistance heating wire having a pair of terminal ends; surrounding said resistance heating wire with an insulating polymeric layer; and forming said polymeric layer and said resistance heating wire into a plurality of turns defining a coil having a central axis, wherein each of said turns is independently elastically expandable to surround a portion of said intravenous tube when said intravenous tube is disposed axially through said coil such that said coil conforms to the shape of said flexible intravenous tube.

10. The method of claim 9, wherein the step of forming said polymeric layer and said resistance heating wire into a plurality of turns includes the steps of wrapping said polymeric layer and said resistance heating wire around a mandrel, heating said polymeric layer and said resistance heating wire, and allowing said thermoplastic sheath to cool to maintain said coil shape.

11. The method of claim 9, wherein said polymeric layer is a tubular thermoplastic sheath and the step of surrounding said resistance heating wire includes the step of disposing said resistance heating wire axially through said tubular thermoplastic sheath.

12. The method of claim 9, wherein the step of surrounding said resistance heating wire includes the step of extruding said polymeric layer over said resistance heating wire.

13. The method of claim 12, wherein the step of surrounding said resistance heating wire includes the step of extruding said polymeric layer over a plurality of resistance heating wires.

14. The method of claim 13, further comprising the step of connecting said plurality of resistance heating wires in parallel.

15. The method of claim 13, further comprising the step of connecting said plurality of resistance heating wires in series.

16. The method of claim 9, where in the step of forming said polymeric layer and said resistance heating wire into a plurality of turns includes the step of forming said polymeric layer and said resistance heating wire into at least two sets of interconnected parallel turns.

17. An expandable heating element, comprising: a resistance heating material surrounded by an electrically insulating polymeric layer, said polymeric layer and said resistance heating material being formed into a plurality of turns defining a coil having an original diameter and a central axis, wherein a plurality of said turns are independently elastically expandable to a diameter greater than the original diameter of said coil to surround a cylindrical body disposed axially through said coil, at least a portion of said cylindrical body having a diameter greater than the original diameter of said coil.

18. The heating element of claim 17, wherein said resistance heating material includes a resistance heating wire having a pair of terminal ends.

19. The heating element of claim 18, wherein said resistance heating wire is laminated between two sheets of thermoplastic.

20. The heating element of claim 18, wherein said polymeric layer is extruded over said resistance heating wire.

21. The heating element of claim 20, wherein said polymeric layer comprises a PTFE film.

22. The heating element of claim 17, wherein said resistance heating material includes a resistance heating wire sewn to a supporting substrate.

23. The heating element of claim 17, wherein said electrically insulating polymeric layer includes a thermoplastic elastomer material, the heating element further comprising a reinforcing substrate fused with said electrically insulating polymeric layer.

24. A method of manufacturing a flexible heating element, comprising the steps of: providing a resistance heating material; surrounding said resistance heating material with an electrically insulating polymeric layer; and forming said polymeric layer and said resistance heating material into a plurality of turns defining a coil having a central axis and an original diameter, wherein a plurality of said turns are independently elastically expandable to a diameter greater than the original diameter of said coil.

25. The method of claim 24, wherein the steps of surrounding said resistance heating material includes the steps of providing a first and second thermoplastic sheets and laminating said resistance heating material between said first and second thermoplastic sheets.

26. The method of claim 25, wherein said forming step includes the steps of wrapping said polymeric layer and said resistance heating material around a mandrel, heating said polymeric layer and said resistance heating material, and allowing said polymeric layer and said resistance heating material to cool to maintain said coil shape.

27. The method of claim 26, wherein said resistance heating material includes a resistance heating wire having a pair of terminal ends.

28. The method of claim 27, wherein said resistance heating wire is sewn to a supporting substrate.

29. The method of claim 24, wherein the step of surrounding said resistance heating material includes the step of extruding said polymeric layer over said resistance heating material.

30. The method of claim 29, wherein the step of forming said polymeric layer and said resistance heating material into a plurality of turns includes the steps of wrapping said polymeric layer and said resistance heating material around a mandrel, heating said polymeric layer and said resistance heating material, and allowing said polymeric layer and said resistance heating material to cool to maintain said coil shape.

31. The method of claim 30, wherein said resistance heating material includes at least one resistance heating wire.

32. The method of claim 24, wherein the step of forming said polymeric layer and said resistance heating material into a plurality of turns includes the steps of wrapping said polymeric layer and said resistance heating material around a mandrel, heating said polymeric layer and said resistance heating material, and allowing said polymeric layer and said resistance heating material to cool to maintain said coil shape.